Summary The long term goal of this project is to gain insight into the nature and role of antibodies in health and disease. B1 cell-derived natural antibodies play a critical homeostatic role in eliminating apoptotic debris and noxious molecules, beyond their role in microbial defense. In mice, B1 cell-derived IgM against atherogenic oxidized low density lipoprotein (oxLDL) protects against heart disease, whereas B2 cell- derived IgG anti-oxLDL exacerbates heart disease. In humans, IgM anti-oxLDL antibodies correlate with atheroprotection, but these antibodies could not be connected to B1 cells, because the definition of human B1 cells was unclear. Moreover, pathological oxLDL represents a heterogeneous collection of molecules. These issues have been addressed with new findings that establish the foundation for the current proposal. 1. We redefined human B1-like cells as CD20+CD27+CD43+ and showed by multiple criteria they are a unique and distinct population. 2. Bengtsson and colleagues focused attention on a specific peptide of apolipoprotein B100 (peptide 220) and showed that the serum level of IgM antibodies against the oxidatively modified peptide is inversely and independently correlated with subsequent cardiovascular events. 3. We showed in collaboration with Bengtsson that beneficial IgM anti-MGO-p220 antibodies are generated by human B1-like cells. 4. Bengtsson and colleagues showed that the serum level of IgG anti-MGO-p220 does not correlate in any way with subsequent cardiovascular events. These results highlight an unresolved issue concerning interaction between the serological arm of the immune system and atherosclerotic heart disease: why do IgM antibodies against oxLDL oppose atherosclerosis but IgG antibodies against oxLDL exacerbate atherosclerosis or do nothing. We propose to address this issue by examining the nature of IgM vs IgG anti-MGO-p220 antibodies. To elucidate the reason why antigen-specific IgM antibodies are beneficial and antigen-specific IgG antibodies are inconsequential, we will: SA1. Obtain IgM and IgG anti-MGO-p220 antibodies for structural comparison, by sorting single cells and PCR amplifying expressed immunoglobulin for sequence analysis. SA2. Express native and mix-and-match IgM vs IgG antibodies, by cloning each antibody as both IgM and IgG, and expressing IgM with and without J chain. SA3. Test the function of IgM vs IgG antibodies, by examining the ability of cloned/expressed antibodies to inhibit macrophage uptake of MGO-modified apoB100. Results of these studies will greatly increase knowledge regarding the mechanism by which IgM natural antibodies fulfill a beneficial role in atherosclerosis and IgG antibodies fail to do the same. The information generated by this work is likely to suggest new therapeutic options that involve administration of, or immunization for, specific antibodies as identified herein.